Cathodic electrodeposition of a film composed of an amine resin, crosslinker, pigment and other resinous components onto an electrically conductive article is an important industrial process. It constitutes the usual manner in which automobile, truck, and bus bodies as well as other large metallic surfaces are primed with paint. In addition to providing a painted surface, the resin systems employed protect the underlying metal surface from corrosion, impact damage and other detrimental exposure to environmental conditions.
In performing the electrodeposition, the conductive article forms one electrode of a direct current circuit and is immersed in a coating ba made from an aqueous dispersion of the film-forming resin and other components. A direct electrical current is passed between the article and a counter-electrode contained in the electrodeposition bath. An electrical charge on the article causes the deposition of the resins and other components of the bath on the article so as to produce an electrodeposited film. The deposited film is then baked or otherwise hardened to yield a coating of substantially uniform thickness and protective characteristics.
Generally, protection from the environment and other adverse conditions is accomplished by designing into the coating resins such chemical characteristics as adhesion, flexibility, strength, hardness, and inertness toward reaction with environmental chemicals. Each of the characteristics manifests itself in the protective properties of the hardened coating.
A number of advances in the protective properties of electrodeposition systems have been described in the patent literature. For example U.S. Pat. Nos. 4,486,571; 4,513,125; 4,565,852; 4,617,348; 4,639,493; 4,657,979; and 4,720,523, the disclosures of which are incorporated herein by reference, describe various dienemodified epoxy resins or cross-linking agents designed for the improvement of the properties of electrodeposition systems.
One particular problem of electrodeposited films relates to impact and chip resistance. The electrodeposited film provides a first line of defense against corrosion of the underlying metal substrate, while simultaneously withstanding impact damage which results in chipping of the overlying decorative layers of coatings as well as delamination of the electrodeposited film from the underlying metal surface. The difficulty lies in chemically designing an electrocoat composition which maximizes both desired effects. Electrodeposited films which possess the requisite flexibility to withstand the shock of impact generally do not, at the same time, possess the required corrosion resistance nor adhere well to overlying layers of decorative coating and are thus more susceptible to chipping. Conversely, electrodeposited films which possess the desired adherence to other coating layers often are hard and do not respond well under impact.
It is therefore an objective of the present invention to provide a principal resin system for use in electrodeposition coating which possesses both high corrosion resistance while simultaneously providing improved impact strength and excellent adherence to the substrate and conventional top coating films.